Hi everyone,

I want to talk a little bit about carb tuning, I am new at it and I am looking for some advice from some of the more experienced people out there. I have an edelbrock 500cfm manuel choke carb on my long rod slant build up and I am in the process of tuning it now. To start I set my idle speed at about 700rpm and turned one of the idle mixture screws in until the engine stumbled and then I turned it back 3/4 of a turn. I repeated that with the other screw as well. I then adjusted my idle speed again to about 700rpm. The engine starts up with an easy turn of the key and idles great but driving is another story. I have a huge bog in acceleration once I get to about 20mph and if I try to go full throttle it stumbles even more. I also can see a slight mist of gas spraying out of the carb when I open the throttle all the way. The acceleration is not 'crisp'. I know I don't have it tuned properly and I will keep tinkering with it. I was just curious how other people have calibrated this carb on a built motor so I can get some ideas on how to tune mine. Such as which jets and metering rods you are using and so on. Thanks

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73' Duster 225
<img src="http://longrod.moparpages.com/enginedone.JPG" width=200>

I'd say getting a Carter/Edelbrock tuning/rebuild book would be a good start so you know what to modify and why (and have a list of parts available).

I'd also install a set of O2 sensors, bungs and A/F ratio meters so you can watch to see if it's lean/rich/etc... and a vacuum gauge. The meters and the vac. gauge cut my time in 'dialing in' my holley tremendously. And currently it helps to 'tech' other issues (like having to rejet due to hot dry weather vs. my 'winter' jetting).

As the engine gets 'worn in', and you work up your timing/distirbutor curve you'll be changing the carb settings a bit again, especially if you run a 'performance' curve vs. the 'standard' distributor curve...


good luck, the fun is just beginning...

-D.Idiot

I recently posted the following information on slantsix.com. My carb is a 625 CFM Carter AFB (now made by Edelbrock) and I was having a similar problem. I believe the problem is due to my lack of intake manifold heat because I was running headers previously and now running Dutra Duals. You didn't mention which manifold you were using and if you were getting any intake manifold heat.

If you are using a Clifford intake, you can easily add heat to the manifold with water from the heater. If you are using an Offenhauser with a stock exhaust manifold, make sure that the heat riser valve is in working order.

When the throttle is quickly cracked open at low RPMs, the low vacuum and air flow cause the fuel to drop out of the air, thereby creating a lean condition. Extra heat would prevent the fuel from condensing on the manifold walls. Since I don't yet have a way of heating my intake manifold, I had to make changes to the fuel mixture.

After trying the a number of metering rod and jet combinations from my Strip Kit, I found that the metering rods with the largest difference between the economy step and the power step seem to work best. Since Federal Mogul is no longer making AFBs and Edelbrock has started making a new and improved AFB, I checked out their metering rods. Edelbrock has some metering rods with even larger differences between their economy and power steps so I tried the next size-difference larger. The performance of my car improved once again and the bog has largely disappeared. To completely eliminate remnants of the bog, there is a still a larger-difference rod available. I wanted to avoid increasing jet sizes because that would affect my cruising fuel economy and I think there is room to make the cruising fuel mixture a bit leaner.

If you are using an Edelbrock or Carter AFB and are experiencing a bog at low RPMs, try experimenting with the following metering rods:

PN .......... Size Difference ........ Cruise/Power Size

16-241 .... 0.0230 .................... 0.0705 x 0.047 (Carter)
16-690 .... 0.0275 .................... 0.0745 x 0.047 (Carter)
#1459 ..... 0.0280 .................... 0.075 x 0.047
#1450 ..... 0.0280 .................... 0.070 x 0.042
#1455 ..... 0.0310 .................... 0.073 x 0.042
#1419 ..... 0.0330 .................... 0.075 x 0.042
#1449 ..... 0.0330 .................... 0.070 x 0.037
#1454 ..... 0.0360 .................... 0.073 x 0.037
#1458 ..... 0.0380 .................... 0.075 x 0.037

To maintain your cruising fuel economy, make your rod changes with rods of the same economy step. You can maintain a fuel mixture for a given rod step (economy or power) and change the fuel mixture of the other step by maintaining a constant flow area between the metering step and the jet. Working out flow areas with a spreadsheet greatly simplifies the selection of metering rods and jets.

For example, if you are getting good fuel economy with MR#1459 and you are using Jet#1429 (0.101), your cruising flow area is 359.4 square inches (to the –5 power). If you want to try using MR#1455, the closest flow area is with Jet#1428 (0.100) with a new cruising flow area of 366.9. Jet#1429 and MR#1459 yield a power flow area of 627.7. Using Jet#1428 with MR#1455, gives a power flow area of 646.9. In this case, you increased your economy flow area by 2.1% while you increased your power flow area by 3.1%.

However, if you are using Jet#1429 and MR#1459 and you want to try MR#1419 with the same jet, your power flow area now increases by 5.6% with no change in economy flow area.

If are using MR#1458 and you still have a bog, you need to increase jet sizes until the bog disappears. However, if this large size-difference metering rod causes your car to run lean when cruising, you will need to increase the primary jet until you no longer feel the surging of a lean fuel mixture or find another metering rod and jet combination that maintains the power flow area while increasing the economy flow area.

Once you have a metering rod and jet combination that works, you have to finetune the acceleration circuit by adjusting the location of the accelerator pump linkage in the pump holes.

The changes I have described above affect only the primary barrels. The secondary barrels will not start flowing on a slant six until the RPMs are high. Once you find a metering rod and jet combination that gives you good drivability, you can then start making changes to the secondary jets. These changes are probably best done at a drag strip or a chassis dynamometer.

In case anyone is wondering, when I was thinking about a performance upgrade for my car several years ago, I also considered the Super Six route, the Holley 500 CFM 2bbl, and Holley 390 4bbl. There was no Proline and Doug hadn’t yet made the reproduction Hyper Pack. I decided in the end that the Carter AFB was what I was going to try because the primary barrels had approximately the same flow capacity as the BBD and the air valves on the secondary barrels would allow more air if required. Setting up the metering rods and jets seemed to be more straight forward that the Holley 390. It also helped that someone had a scrap 625 CFM AFB lying around that I could have for free. After thoroughly reading the Direct Connection Racing Manuals , I was concerned about fuel distribution with the 4bbl carb. I decided to machine a new baseplate to mount the carb sideways with the throttle shafts parallel to the engine. From what I can tell from the spark plugs, my fuel distribution seems to be symmetrical. I have also eliminated the bog on left turns by raising float level.

BTW, when you adjust the idle fuel mixture screws, start with the screws turned out exactly the same amount of turns. Then, turn both screws exactly the amount in or out until your idle is as smooth and high as possible.

Does anyone else have similar (or different) experiences with setting up their AFB?

Frank

Now there's some information for you!

I've never run the 500 AFB on a /6, so I can't give you specifics. What Frank sez should give you an idea what to do. Here's a good starting point for jets/rods on a small AFB on a /6:

Primary Jets: .089"
Secondary Jets: .062"
Metering Rods: .068 X .062"

I don't even remember what's in the 400 AFB on my /6 anymore, it's been so long since I messed with it. I could have written a book for you way back when. I do remember I ended up staggering the secondary jets (inboard jet is smaller than outboard jet).

A couple (well, four ) things. One, just a little nitpick of no consequence, Edelbrock does not make the AFB, they just sell it. It is made under license by Weber. I don't even know if you can still get the Carter marketed one (also made by Weber), but they are the same carb. Most parts for the newer AFB/AVS including jets, rods and pump shooters are interchangable with older OEM Carter carbs as well, just remember 2 step rods go with the regular jets (which are the same design primary and secondary), the three step rods (found on some OEM AFB/AVS's) require raised jets, special metering rod pistons and dimpled metering rod covers. For simplicity's sake, I'd stay away from those.

Two, it would probably help you to obtain a strip kit which contains a bunch of rods and jets. They can be had new or find someone selling a used one on-line. The only problem is that most of the stuff is bigger and geared toward an AFB on a V8, so you may end up having to solder some jets and redrilling with a precision drill bit, particularly to get the smallish secondary jets. You can also "make" your own custom metering rods if you have to - I just work them down a little by twisting them in some fine sandpaper, being careful to try to make them even, then I check everthing with a set of calipers.

Third, looking at the "new" Edelbrock AVS, I don't see why you couldn't get an AVS airhorn and put it on a 500 AFB (removing the weighted door), thereby having a 500 AVS. I think an old AVS top might actually work as well, but the air door would have to be notched for the AFB's booster venturis on the secondaries. This would be far superior in secondary airflow adjustability for the /6, but please note that I have not actually tried this yet.

Lastly, I offer the following. I'm gonna give away one of my secrets here, so everyone pay attention! I solved the cold engine driveability issue on my 4V duster (which was much worse with a Holley, I might add) by deploying a 30 X 30 mesh, .065 wire stainless steel screen under the carb. I did this by placing it between two gaskets with a bead of fuel-resistant RTV under the adapter on my Offy intake (which all the Offy four-barrel /6 intakes have). This puts it out of the way of the butterflies. I later went back and cut it out under the secondaries since off idle driveability is not a factor when you are romping on one, and it is a slight flow restriction. There you have it.

D/W

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If it ain't broke, fix it!

My grandfather did this on his Chrysler 300 with the 383-4v original AFB when he was super tuning it. Was not necessary with the Holley carb he used later, but it did help make things 'right' for in town driving with the AFB, and made the emissions better when sniffed by an Exhaust Gas Analyzer. I still have his 'sandwiches' and meshed gaskets on my garage pegboard wall for 'show and tell'....

Good trick Dennis!


-D.Idiot

Here's a pic of Dennis's 'trick' off my wall, worked good when my grand father made his own gasohol after Ronnie Reagan 'deregulated' homebrew fuel production in 1980-1981....





Here's a pic of the O2 sensor in my front dutra dual, and a pic of the A/F meters under my dash (just turned the key on, chokes closed, and the engine's not running so they're rolled over to 'rich', they come off that after the mill starts...)








-D.Idiot

We used to call that an "Oklahoma Supercharger". (No offense intended to any Sooners out there.)
I ran one for a while on my '63 Belair 230-inch six, on the theory that the turbulence improved mix. (Ok, I was in high school and tried lots of cheap tricks intended to get a little more oomph.) I never noticed any performance difference, but it did catch a dropped air cleaner wing nut once, possibly saving the engine ...

Thanks for the info.
D.I. - why do you have two O2 sensors? Is it because of the dual exhaust? Would/do you get a different reading from each side?

Frank - How big is the primary jet in your carb? I have a 500 cfm, I think you said you're using a 625.

Dennis - You wrote a .062 jet for the secondary. I just wanted to double check, should that be .092? I thought the secondary should be larger than the primary.
Thanks again for helping "speed up" my dial in.

_________________
"Nothing in all creation is hidden from God's sight. Everything is uncovered and laid bare before the eyes of Him to whom we must give account."

My AFB currently has the following installed in it:
Primary Jet 120-401 (0.101")
Metering Rod 16-7542 (Edelbrock #1419) (.075" x .042")
MR Spring 61-604 (9 oz)
Secondary Jet 120-380 (0.080")

It is a 625 CFM model and the difference between it and your 500 CFM model is that the secondary venturi is larger by 5/16". Everything else seems to be identical. If you are using your Edelbrock on a stock engine, the jets and rods I was using may not work perfectly for you because I have a nonstock cam, larger valves, and a larger exhaust system. Also, after installing the Dutra Duals, I had to richen the fuel mixture and I only did enough to make the car drivable. I didn't spend any time trying to optimize the economy mixture or get the secondary mixture right.

Frank

Nope. No typo. Remember, the primary jet has a rod sticking in it at all times, so the effective area is actually less.

D/W

_________________

If it ain't broke, fix it!

That sure seems like a rich setup. But hey, if it works for you... What cam are you running?

D/W

_________________

If it ain't broke, fix it!

You are right. I had to make the power step richer to eliminate the off-idle bog I was getting. As the power mixture got richer, the bog became smaller. I knew this was way richer than what the slant six needed, which confirmed to me that my intake manifold was too cold. I figured that the problem was not that the carb was oversized because I could see that there was fuel flowing from the primary venturis as I gunned it in my garage. I didn't spend any time trying to find the metering rod & jet combination that gave me the best fuel economy now that I've installed the Dutra Duals.

If I was to continue using the AFB, I am sure that I would have had to lean out the power fuel mixture once the intake heater was installed. I had a similar off-idle bog with the Quadrajet that disappeared after I installed the heater.

To compensate for the fuel dropping out when using an unheated manifold on the street, I still recommend using the Edelbrock rods with the large difference in size between the economy and power steps.

As for the cam I'm using, it's a Direct Connection high lift, short duration cam (PN 4120243). I decided to use this one after reading up on cam choices in the Slant Six Racing Manual. I am using it with the stock automatic transmission and 2.93 gears and I'm pretty happy with the way it works.

Frank

Hmmm, that's an interesting test. You do realize that there would be a big difference in flow in a loaded vs. non-loaded engine, right? Still, I think the air door will keep the "big bog" away on a slant 6 even with super-large secondaries. I do think one must keep the primary size pretty small if off-idle drivability is a concern.

D/W

_________________

If it ain't broke, fix it!

Actually, I'm quite aware that a loaded engine will require more air than an unloaded one. That's why I reasoned that if there if fuel flowing through the main circuits when the transmission is in park and the engine is revving, there should be more flow when the engine under load on the road. If the primary venturis are too big, there should be minimal fuel flow from the main circuits until the airflow (and RPMs) is high enough to start it. My understanding is that the air doors should prevent or minimize any airflow through the secondaries until the engine requires the extra air.

The first part of my troubleshooting effort was trying to eliminate a bog when I quickly cracked the throttle open and closed while the engine was idling. I could see that in doing this, there was always fuel flow through the primary venturis. Trying different accelerator pump settings seemed to have minimal effect. Richening the power step did improve the smoothness of the increase in RPMs when I cracked open the throttle.

The second thing I had tried to troubleshoot this problem was to find a straight section of highway near my house without too much traffic. I would stop the car, put one foot on the brake and the other on the gas. (What does everyone call this anyway? -- brake torque, brake stand?) As I slowly increased the pressure on the gas pedal, I noticed that the engine would lose power before the throttle was wide open. I couldn't be sure that the secondary throttle valve (as opposed to the air door) hadn't started to open but I'm pretty sure the power loss occurred before it had. I found that as the primary power step got smaller relative to the size of the economy step, the loss of power got less as well.

I would have tried doing this in my garage with my wife doing the gas pedal and brake thing so I could verify flow through the secondary venturis but the floor is too smooth and the tires always start to spin no matter what the jetting.

I think you are right about the small primaries helping to keep the off-idle bog down but I think the reason it succeeds is because it keeps manifold vacuum from dropping too low. I think higher vacuum helps keep the fuel vaporized. The small barrels of the Holley 390 probably keep the air speed up to help keep the atomized fuel in suspension. The downside of having a higher manifold vacuum at full throttle is that it lowers the engine's volumetric efficiency somewhat.

Like I said before, I had a similar but smaller bog with the Quadrajet but it disappeared after I added heat to the intake manifold. BTW, the Quadrajet has smaller primaries than the BBD and the Holley 390 and the spring-loaded air valve appears to do a better job of sealing off the secondary barrels than the air doors of the AFB.

Frank

Just the opposite, really. Remember, load is holding the crank rotation back, therefore the air velocity will increase at a much slower rate than when the motor is revved in neutral . The total air-flow potential of your engine would be the same, of course.
You nailed it with the last part. It is the air velocity through the smaller venturies that helps.

Another oft neglected aspect with tuning the accelerator pump circuit is shooter size. The pump displacement/travel affects the overall volume of the squirt, the shooter size affects the timing, in other words, a bigger shooter will dump a bunch of gas out all at once and a smaller one will sqirt a thin stream for a longer period of time. It usually takes fiddling with both to really get things optimized.

D/W

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If it ain't broke, fix it!